Sensing light emitted from multiple light sources

ABSTRACT

An apparatus is directed to controlling a light source. The apparatus provides at least one light source that emits a light signal at a discrete frequency and a reference signal at the discrete frequency. The apparatus further includes a photodetector optically coupled to the light source and designed to receive the light signal. The apparatus additionally includes at least one lock-in system coupled to the photodetector and each light source that receives the light signal from the photodetector and receives the reference signal from the light source. Each lock-in system produces an intensity value of the light source based on the light signal and the reference signal. The lock-in system may include a frequency multiplier and a filter coupled to the frequency multiplier wherein the intensity value is the product of the light signal and the reference signal processed through the frequency multiplier, and filtered to remove non-dc portions.

The technical field of this disclosure is light production from lightemitting diodes (LEDs), particularly, sensing light emittedsimultaneously from multiple light sources.

Illumination sources, such as lamps, currently utilize incandescent andfluorescent means as light production. It is well known thatincandescent light sources are inefficient light sources that utilizemore power resources than other light sources. Fluorescent light sourceshave provided a more efficient light production.

Light emitting diodes (LEDs) produce light in a much more efficientmanner than incandescent light sources, but until recently have not beenmanufactured in a cost efficient manner to utilize in lightingapplications. Expectations are for LEDs to produce light moreefficiently than fluorescent light sources in the near future. Recently,LED production has made utilizing LEDs in light production applicationsa viable alternative.

Producing usable light with LEDs generally requires either manufacturingan LED that produces a specified color, such as utilizing a phosphorlayer overlying the LED, or mixing a plurality of colored LEDs toproduce a desired colored light output. Unfortunately, once a lightsource package is produced to achieve the desired colored light outputits useful life is reduced to the amount of time until a failure orpartial failure of one of its component parts occurs.

Unfortunately, LED characteristics depend on temperature, drive current,and time. Additionally, LED characteristics vary from LED to LED.Although an LED-based lamp may be set to operate at a given color pointand intensity, at the beginning of its life, the actual color andintensity obtained at that setting may not remain constant.

Mixing a plurality of colored light sources may include a control systemthat varies individual light source contributions to correct forvariation in the LED characteristics That is, as the output of componentLEDs varies, the control system can maintain a desired spectral outputand intensity by varying individual LED output to compensate for thevariations.

Currently, sensing systems for controlling a specified colored lightoutput include temperature feed-forward or intensity feedback systemscontaining a single unfiltered photodiode. Another sensing systemincludes utilizing multiple photodiodes, for example three or more, andcorresponding color filters. This system may be referred to as a colorfilter photodiode control system.

In one embodiment, this system can be implemented utilizing a time-basedapproach whereby the LEDs are pulsed on and off in a particular patternallowing the sensing of the intensity of the independent LED groups. Anadvantage of the color filter photodiode control system over thetemperature feed-forward or intensity feedback systems is that the colorfilter photodiode control system can sense the average levels of thedifferent spectral outputs of the LEDs, for example red, green, andblue, without having to turn the LEDs on and off in a particularpattern. Additionally, a low pass filter can be used to integrate thesignal from each LED group. The accuracy of this method is stronglyinfluenced by the color filters on the photodiodes.

Unfortunately, as described above, temperature feed-forward or intensityfeedback systems require that LEDs be turned on and off briefly topermit sensing of the individual color components, for example red,green and blue. This approach is susceptible to errors resulting fromripple in the driving current, and changes in the drive waveform, suchas, for example changes in the rise and fall times of the LED drivecurrent pulses. The color filter photodiode control system, although notrequiring the turning on and off of LEDs to sense the individual colorcomponents, does require more expensive sensors containing color filtersas well as a larger total number of sensors. None of the systemscorrects for ambient light.

It would be desirable, therefore, to provide a system that wouldovercome these and other disadvantages.

The present invention is directed to an apparatus and method forcontrolling a light source. The invention provides for a frequencysensing structure that produces an intensity value input for a controlsystem.

One aspect of the invention provides a light source control apparatusincluding at least one light source that emits a light signal at adiscrete frequency and a reference signal at the discrete frequency. Theapparatus further includes a photodetector optically coupled to thelight source and designed to receive the light signal. The apparatusadditionally includes at least one lock-in system coupled to thephotodetector and each light source, to receive the light signal fromthe photodetector and an associated reference signal from the lightsource. Each lock-in system produces an intensity value of the lightsource based on the light signal and the associated reference signal.

In accordance with another aspect of the invention, the inventionprovides a method for sensing intensity of a light source. The methodincludes emitting at least one light signal where the light source isdriven at a discrete frequency. The method further includes transmittinga reference signal associated with each of the light signals at thediscrete frequency. The method additionally includes producing anintensity value based on the light signal and the associated referencesignal.

In accordance with yet another aspect of the invention, the inventionprovides a system for sensing intensity of a light source. The systemincludes means for emitting at least one light signal where the lightsource is driven at a discrete frequency. The system further includesmeans for transmitting a teference signal associated with each of thelight signals at the discrete frequency. Means for producing anintensity value based on the light signal and the associated referencesignal are also included.

The foregoing and other features and advantages of the invention willbecome further apparent from the following detailed description of thepresently preferred embodiment, read in conjunction with theaccompanying drawings. The detailed description and drawings are merelyillustrative of the invention rather than limiting, the scope of theinvention being defined by the appended claims and equivalents thereof.

FIG. 1 is a schematic diagram illustrating a sensing device according toan embodiment of the present invention;

FIG. 2 is a schematic diagram illustrating a portion of the sensingdevice in FIG. 1 according to an embodiment of the present invention;

FIG. 3 is a schematic diagram illustrating another portion of thesensing device in FIG. 1 according to an embodiment of the presentinvention;

FIG. 4 is a schematic diagram illustrating a sensing device according toanother embodiment of the present invention; and

FIG. 5 is a flow-diagram depicting an exemplary method in accordancewith the present invention.

Throughout the specification, and in the claims, the term “connected”means a direct physical or optical connection between the things thatare connected, without any intermediate devices. The term “coupled”means either a direct physical or optical connection between the thingsthat are connected or an indirect connection through one or more passiveor active intermediary devices. The term “circuit” means either a singlecomponent or a multiplicity of components, either active or passive,that are coupled together to perform a desired function.

FIG. 1 is a schematic diagram illustrating a sensing device 100according to an embodiment of the present invention. Device structure100 includes control units (110, 120, and 130), light emitting diodes(115, 125 and 135), a photodetector 150, and lock-in systems (170, 180,and 190). In one embodiment, implementation of the present inventionallows any number of light emitting diodes (LEDs) to be utilized, solong as there is a corresponding control unit and lock-in system foreach LED. In another embodiment, each LED represents a block ofindependently-driven LEDs with a substantially similar spectral lightoutput. For example, LED 115 may consist of several LEDs, all emitting ared light output. Similarly, LED 125 may include all green lightemitting LEDs, and LED 135 may include all blue light emitting LEDs.

In one example, the present invention is implemented as a single LED ora single color group of LEDs, a single control unit, and a singlelock-in unit in addition to the photodetector. In another example andreferring to FIG. 1, sensing device 100 is implemented as a plurality ofLEDs or multi-color LED groups, each independently-driven LED or LEDgroup having an associated control unit and an associated lock-insystem. In this example, emitted spectra of the LEDs form a multi-sourcelight signal. For example, a red, a green, and a blue LED or groups ofLEDs are utilized to produce a “white” multi-source light signal.

Each control unit (110, 120, and 130), detailed in FIG. 2 below,includes an associated output drive signal terminal (Drv1, Drv2, andDrv3) and an associated output reference terminal (Ref1, Ref2, andRef3). Each output drive signal terminal (Drv1, Drv2, and Drv3) iscoupled to an associated light emitting diode (115, 125 and 135).

In an example, output drive signal terminal (Drv1) is coupled to lightemitting diodes (115), output drive signal terminal (Drv2) is coupled tolight emitting diode (125), and output drive signal terminal (Drv3) iscoupled to light emitting diode (135).

Light emitting devices (115, 125 and 135) are optoelectronic devicesthat produce light when power is supplied causing them to forward bias.The light produced may be within the blue, green, red, amber or otherportion of the spectrum, depending on the material utilized inmanufacturing the LED. In an example, LEDs (115, 125 and 135) areimplemented as LXHL-BM01, LXHL-BB01 and LXHL-BD01 available fromLumileds corporation of San Jose, Calif. In another example, LEDs (115,125 and 135) are implemented as NSPB300A, NSPG300A and NSPR800AS fromNichia corporation of Mountville, Pa.

Each control unit produces a drive signal and a reference signal, asdetailed in FIG. 2 below. Power, in the form of the drive signal, istransmitted to the associated light emitting diode (LED) or LED groupand the reference signal is transmitted to the associated lock-in unit.The LED receives the drive signal and produces a light signal based onthe drive signal. The drive signal is generated at a discrete frequency.

The reference signal is transmitted to the associated lock-in system andincludes the same discrete frequency. Multiple control units andassociated LEDs produce a light signal including several intensityvalues representing the intensity of light emitted by each LED or LEDgroup.

It is important to differentiate between the discrete frequency drivingthe light signal emitting from each LED or LED group and the very highfrequency that the LED or LED group emit as light. Typically, asdescribed below, the drive signals range from about 400 Hz to about 1.2kHz while the light emitted from the LEDs or LED groups is in the orderof 10¹⁴ Hz.

Photodetector 150 is an optoelectronic device that responds to lightsignals and produces a received light signal. In one embodiment,photodetector 150 is implemented as a photodiode, such as, for examplePS 1-2CH from Pacific Silicon Sensor, Inc of Westlake village, Calif.Photodetector 150 includes an output signal terminal (Rec) for supplyingthe received light signal.

In one embodiment, photodetector 150 responds to a single source lightsignal and produces a received light signal, at the output signalterminal (Rec), which corresponds to the intensity of light produced bythat single light source. In another embodiment and described in FIG. 5below, photodetector 150 responds to the multi-source light signal andproduces a received light signal, at the output signal terminal (Rec).The received light signal includes components at multiple frequencies,each component corresponding to the intensity of one light source in themulti-source light signal.

Each lock-in system (170, 180, and 190) includes a lock-in device,detailed in FIG. 3 below. Each lock-in system (170, 180, and 190)further includes an input signal terminal (Rec) and an associated inputreference terminal (Ref1, Ref2, and Ref3). Each input signal terminal(Rec) of each associated lock-in system (170, 180, and 190) is coupledto the output signal terminal (Rec) of photodetector 150. Each inputreference terminal (Ref1, Ref2, and Ref3) of each associated lock-insystem (170, 180, and 190) is coupled to the output reference terminal(Ref1, Ref2, and Ref3) of each associated control unit (110, 120, and130).

In an example, output reference terminal (Ref1) of control unit 110 iscoupled to input reference terminal (Ref1) of lock-in system 170, outputreference terminal (Ref2) of control unit 120 is coupled to inputreference terminal (Ref1) of lock-in system 180, and output referenceterminal (Ref3) of control unit 130 is coupled to input referenceterminal (Ref3) of lock-in system 190.

Each lock-in system (170, 180, and 190) further includes an associatedoutput intensity signal terminal (Int1, Int2, and Int3), detailed inFIG. 3 below. Each lock-in system receives an input signal, at the inputsignal terminal (Rec), from photodetector 150 and a reference signal, atthe input reference terminal (Ref1, Ref2, and Ref3), from an associatedcontrol unit (110, 120, and 130). Each lock-in system produces an outputintensity signal, at the associated output intensity signal terminal(Int1, Int2, and Int3), based on the received input signal and referencesignal.

In a further embodiment, sensing device 100 includes a high-pass filtercoupled between the output reference terminal (Ref1, Ref2, and Ref3) ofeach control unit (110, 120, and 130) and the input reference terminal(Ref1, Ref2, and Ref3) of the associated lock-in system (170, 180, and190). In one embodiment, coupling a high-pass filter between the controlunit and the lock-in system reduces spurious dc components fromaffecting the reference signal.

FIG. 2 is a schematic diagram illustrating a control unit 210 accordingto an embodiment of the present invention. Control unit 210 includes afrequency shifter 215, a power distributor 217, an input clock signalterminal (Clk), an input power signal terminal (Pwr), an outputreference signal terminal (Ref), and an output drive signal terminal.Control unit 210 receives a clock signal and a power signal, produces areference signal based on the clock signal, and produces a drive signalbased on the reference signal and the power signal.

Frequency shifter 215 includes an input clock signal terminal (Clk) andan output reference signal terminal (Ref). Frequency shifter 215receives the clock signal and produces the reference signal based on theclock signal. In one embodiment, frequency shifter 215 receives theclock signal and “divides down” the clock signal to produce thereference signal. The reference signal frequency utilized is produced ata frequency so as not to cause noticeable “flicker” to the human eye. Inan example, a reference signal is produced in the 100 Hz-2.4 kHz range.

In another embodiment, frequency shifter 215 includes an internal clockthat generates the clock signal internally thereby eliminating the needfor the clock terminal (Clk).

Additionally and referring to FIG. 1, the use of multiple control units(110, 120, and 130) requires several discrete frequencies. Thefrequencies utilized are produced so that frequency overlap will notoccur. In one embodiment, the frequencies utilized are produced with a100 Hz gap between discrete frequencies. In an example, control unit 110produces a reference frequency at 409 Hz, control unit 120 produces areference frequency at 500 Hz, and control unit 130 produces a referencefrequency at 600 Hz.

Power distributor 217 includes an input power terminal (Pwr), an inputreference signal terminal (Ref), and an output drive signal terminal(Drv). The input reference terminal (Ref) of power distributor 217 iscoupled to the output reference terminal (Ref) of frequency shifter 215.Power distributor 217 receives the power signal and the reference signaland produces the drive signal based on the power signal and thereference signal.

In one embodiment, the power signal is implemented as a voltage sourcesignal. In another embodiment, the power signal is implemented as acurrent source signal. In an example, power distributor 217 produces adrive signal including a current signal modulated at a discretefrequency associated with the reference signal.

The power signal may be produced in the form of one of several differentwaveforms, such as, for example, a sine wave, a cosine wave, a squarewave, or any other waveform that would allow the production of the lightsignal.

FIG. 3 is a schematic diagram illustrating a lock-in device 370according to an embodiment of the present invention. Lock-in device 370includes a signal multiplier 375, a filter 377, an input signal terminal(Rec), an input reference terminal (Ref), and an output intensityterminal (Int). Lock-in device 370 receives an input signal and areference signal, and produces an intensity signal based on the inputsignal and the reference signal.

Signal multiplier 375 includes an input signal terminal (Rec), an inputreference terminal (Ref), and an output product terminal (Prd). Signalmultiplier 375 receives the input signal and the reference signal, andproduces a product signal based on the input signal and the referencesignal. Signal multiplier 375 produces the product signal by multiplyingthe input signal by the reference signal, detailed in FIG. 5 below.Signal multiplier 375 can be implemented as a signal multiplier chip,such as, for example the MLT04 produced by Analog Devices of Norwood,Mass.

Filter 377 includes an input product terminal (Prd) and an outputintensity terminal (Int). The input product terminal (Prd) of filter 377is coupled to the output product terminal (Prd) of signal multiplier375. Filter 377 receives the product signal and filters the receivedproduct signal to remove non-dc portions of the signal. In oneembodiment, filter 377 is implemented as a low-pass filter.

FIG. 4 is a schematic diagram illustrating a sensing device 400according to another embodiment of the present invention. Devicestructure 400 includes control units (110, 120, and 130), light emittingdiodes (115, 125 and 135), photodetectors 450 and 455, and lock-insystems (470, 480, and 490). Like components from FIG. 1 are numberedidentically and function identically. In one embodiment, implementationof the present invention allows any number of light emitting diodes(LEDs) to be utilized, so long as there is a corresponding control unitand lock-in system for each independently driven LED or group of LEDs.

Photodetectors 450 and 455 are optoelectronic devices that respond tolight signals across the whole visible spectrum, and each produce areceived light signal within a predetermined spectrum. In oneembodiment, photodetectors 450 and 455 are implemented as two separatesingle junction photodiodes, such as, for example PSS 1-2CH from PacificSilicon Sensor, Inc. In this embodiment, photodetector 450 includes anoutput signal terminal (Rec1) for supplying a portion of the receivedlight signal, and photodetector 455 includes an output signal terminal(Rec2) for supplying another portion of the received light signal.

In another embodiment, photodetectors 450 and 455 are implemented as amulti-junction photodiode, such as, for example PSS-WS7.56 from PacificSilicon Sensor, Inc. In this embodiment, photodetector 450 represents afirst junction of the multi-junction photodiode, and photodetector 455represents a second junction of the multi-junction photodiode. Onejunction is more sensitive to red wavelengths, and the other is moresensitive to blue wavelengths. Comparison of the measurements of the twojunctions provides a measure of spectral shift.

In an example, photodetector 450 responds more strongly thanphotodetector 455 to light signals within the spectrum defined asgreater than about 600 nm. In this example, photodetector 455 respondsmost strongly than photodetector 450 to light signals within thespectrum defined as less than about 600 nm.

Photodetectors 450 and 455 respond to single and multi-source lightsignals and produce a received light signal, at the output signalterminals (Rec1 and Rec2). In one embodiment, each received light signalincludes single or multiple intensity values. In this embodiment, eachintensity value includes a discrete frequency.

In another embodiment each received light signal includes components atsingle or multiple frequencies. In this embodiment, each componentcorresponds to the intensity of one light source in the multi-sourcelight signal.

Each lock-in system (470, 480, and 490) includes multiple lock-indevices (475, 477, 485, 487, 495, and 497), each lock-in devicefunctions as described in FIG. 3 above. In one embodiment, the number oflock-in devices within each lock-in system is equal to the number ofphotodetectors. In an example, lock-in devices (475, 485, and 495) arecoupled to photodetector 450 via input signal terminal (Rec1), andlock-in devices (477, 487, and 497) are coupled to photodetector 455 viainput signal terminal (Rec2).

Each lock-in system (470, 480, and 490) further includes associatedinput reference terminals (Ref1, Ref2, and Ref). Input referenceterminals (Ref1, Ref2, and Ref3) of each associated lock-in system (470,480, and 490) are coupled to the output reference terminal (Ref1, Ref2,and Ref3) of each associated control unit (110, 120, and 130). In anexample, output reference terminal (Ref1) of control unit 110 is coupledto each input reference terminal (Ref1) of lock-in devices (475 and 477)within lock-in system 470. Output reference terminal (Ref2) of controlunit 120 is coupled to input reference terminal (Ref1) of lock-indevices (485 and 487) within lock-in system 480. Output referenceterminal (Ref3) of control unit 130 is coupled to input referenceterminal (Ref3) of lock-in devices (495 and 497) within lock-in system490.

Each lock-in device (475, 477, 485, 487, 495, and 497) further includesmultiple output intensity signal terminals (Int1/1, Int2/1, Int1/2,Int2/2, Int1/3, and Int2/3). In one embodiment, the number of outputintensity signal terminals within each lock-in system is equal to thenumber of lock-in devices, and therefore is equal to the number ofphotodetectors.

Each lock-in device receives a portion of the received light signal froman associated photodetector, and receives a reference signal from anassociated control unit. Each lock-in system produces an outputintensity signal, at the associated output intensity signal terminal(Int1/1, Int2/1, Int1/2, Int2/2, Int1/3, and Int2/3), based on thereceived input signal and reference signal.

In a further embodiment, sensing device 100 includes a high-pass filtercoupled between the output reference terminal (Ref1, Ref2, and Ref3) ofeach control unit (110, 120, and 130) and the input reference terminal(Ref1, Ref2, and Ref3) of the associated lock-in system (470, 480, and490). In one embodiment, coupling a high-pass filter between the controlunit and the lock-in system reduces spurious dc components fromaffecting the reference signal.

FIG. 5 is a flow diagram depicting an exemplary method for sensingintensity of a light source in accordance with the present invention.Method 500 may utilize one or more systems detailed in FIGS. 1-4, above.

Method 500 begins at block 510 where a control system for a light sourcedetermines a need to sense the intensity of one or more light emittingdiodes (LEDs) or groups of LEDs within the light source. Method 500allows the control system to determine power requirements for each LEDby providing the control system with an intensity value for each LED orgroup of independently-driven LEDs. Method 500 then advances to block510.

At block 510, the light source emits a light signal. Referring to FIGS.1 and 2, the light source includes at least one light emitting diode(LED) or group of LEDs, each independently-driven LED or group of LEDsemitting a light signal that includes an intensity value within theLED's spectral band, and being driven with a current waveform at adiscrete frequency.

In an example, the light source includes three LEDs or groups of LEDs,each LED or group of LEDs coupled to and receiving a drive signal froman associated control unit (110, 120, and 130), and combining to producea “white” light output. That is, LED (115) is driven with an AC currentat a frequency ω_(R) and emits light in the red spectrum, LED (125) isdriven with an AC current at a frequency ω_(G) and emits light in thegreen spectrum, and LED (135) is driven with an AC current at afrequency ω_(B) and emits light in the blue spectrum. For illustrativepurposes, a cosine waveform is utilized. The resulting light signal isthen expressed as:A_(R) cos ω_(R)t+A_(G) cos ω_(G)t+A_(B) cos ω_(B)twhere A is the magnitude and ω is the frequency of the associatedsignal.

In this example control unit (110) and LED (115) produce the A_(R) cosω_(R)t component, control unit (120) and LED (125) produce the A_(G) cosω_(G)t component, and control unit (130) and LED (135) produce the A_(B)cos ω_(B)t component. In this example and referring to FIG. 1, the redLED (115) is driven at 400 Hz (ω_(R)), the green LED (125) is driven at500 Hz (ω_(G)), and the blue LED (135) is driven at 600 Hz (ω_(B)).

In one embodiment, a square wave is utilized as the waveformcharacteristics include the ability to set the lower portion of thewaveform to zero amps. The ability to set the lower portion of thewaveform to zero is important as it allows for cancellation ofundesirable components during production of an output intensity signal.

In one embodiment and referring to FIGS. 1 and 3 above, the light signalis received by photodetector 150 and transmitted to each lock-in system(170, 180, and 190) as the received light signal. In another embodimentand referring to FIGS. 3 and 4 above, the light signal is received byphotodetectors 450 and 455, and transmitted to each lock-in system (470,480, and 490) as the received light signal.

In this embodiment, a portion of the received light signal, received byphotodetector 450, is transmitted to one lock-in device (475, 485, and495) within each lock-in system (470, 480, and 490). Additionally,another portion of the signal, received by photodetector 455, istransmitted to the other lock-in device (477, 487, and 497) within eachlock-in system (470, 480, and 490). Method 500 then advances to block520.

At block 520, the control unit transmits a reference signal to anassociated lock-in system. In one embodiment and referring to FIG. 1,each control unit (110, 120, and 130) transmits an associated referencesignal to an associated lock-in system (170, 180, and 190). In thisembodiment, each reference signal is produced by the associated controlunit and transmitted at a discrete frequency.

In an example and referring to FIGS. 1 and 2, control unit 210 receivesthe clock signal and produces the reference signal based on the clocksignal. Alternatively and detailed in FIG. 2 above, the frequency couldbe generated internally within each controller thereby negating the needfor an external clock. Additionally, for illustrative purposes a cosinewaveform is utilized. The resulting reference signal is then expressedas:I_(ref) cos ω_(ref)twhere I_(ref) is the magnitude and ω_(ref) is the frequency of thereference signal.

In this example, the reference signal produced by control unit 120 isexpressed as:I_(ref) cos ω_(G)t

The reference signal is then transmitted to each lock-in system. In oneembodiment, the reference signal is transmitted to each lock-in system(170, 180, and 190) as the reference signal of FIG. 1, above. In anotherembodiment, the reference signal is transmitted to each lock-in system(470, 480, and 490) as the reference signal of FIG. 4, above. Method 500then advances to block 530.

At block 530, the lock-in system produces an intensity value based onthe received light signal and the associated reference signal. In oneembodiment and referring to FIG. 1, each lock-in system (170, 180, and190) receives the received light signal from photodetector 150 andreceives an associated reference signal from an associated control unit(110, 120, and 130).

In an example and referring to FIGS. 1 and 3, signal multiplier 375 oflock-in device 370 receives the received light signal and the associatedreference signal. In this example, signal multiplier 375 produces aproduct signal by multiplying the received light signal by theassociated reference signal. The resulting product signal is thenexpressed as:I_(ref)*A_(R) cos ω_(ref)t cos ω_(R)t+I_(ref)*A_(G) cos ω_(G)t*cosω_(R)t+I_(ref)*A_(B) cos ω_(B)t*cos ω_(R)tmultiplication of the cosine terms results in the product signalexpressed as½I_(ref)*A_(R) cos(ω_(ref)−ω_(R))t+½I_(ref)*A_(R)cos(ω_(ref)+ω_(R))t+½I_(ref)*A_(G) cos (ω_(ref)−ω_(G))t+½I_(ref)*A_(G)cos(ω_(ref)+ω_(G))t+½I_(ref)*A_(B) cos(ω_(ref)−ω_(B))t+½I_(ref)*A_(B)cos(ω_(ref)+ω_(B))t

In this example and described above, lock-in device 370 represents thelock-in device within lock-in system 180 of FIG. 1, above. Therefore,the resulting reference signal is produced by control unit 120 andexpressed as:I_(ref) cos ω_(ref)t=I_(ref) cos ω_(G)tsubstitution results in the product signal expressed as:½I_(ref)*A_(R) cos(ω_(G)−ω_(R))t+½I_(ref)*A_(R)cos(ω_(G)+ω_(R))t+½I_(ref)*A_(G)+½I_(ref)*A_(G) cos2ω_(G)t+½I_(ref)*A_(B) cos(ω_(G)−ω_(B))t+½I_(ref)*A_(B)cos(ω_(G)+ω_(B))t

In this example, the product signal is then transmitted to filter 377.Filter 377 is implemented as a low-pass filter having a cutoff frequencythat discards the non-dc terms. The cutoff frequency must be less thaneither (ω_(G)−ω_(R)) or (ω_(G)−ω_(B)), for example, below 100 Hz whenutilizing the above example frequencies. The result of filtering theproduct signal is removal of the non-dc terms and is expressed as:½I_(ref)*A_(G)

In this example and referring to FIGS. 1 and 3, the resulting signal isthe intensity value. The reference intensity value may be removed, forexample, by “dividing” it out. Alternatively, an unaltered intensityvalue can be returned to the control system.

In another embodiment and referring to FIG. 4, each lock-in system (470,480, and 490) receives the received light signal from photodetectors 450and 455, and receives an associated reference signal from an associatedcontrol unit (110, 120, and 130). In this embodiment, one lock-in deviceof each lock-in system, for example lock-in device 485 of lock-in system480, receives a portion of the received light signal. The second lock-indevice of each lock-in system, for example lock-in device 487 of lock-insystem 480, receives another portion of the received light signal. Eachlock-in device (485 and 487) produces a component intensity value at theassociated intensity signal terminal (Int1/2, Int2/2), as describedabove. In an example, the component intensity values are summed toproduce a single intensity value for the associated spectrum (e.g.Green). In an example, the ratio of the two components values provides ameasure of any spectral shifts that may have occurred during lightsource operation. Method 500 then advances to block 550, where itreturns the intensity values to the control system.

The control system utilizes the intensity values to determine the amountof power to supply to the LEDs of the light source. In one embodimentand referring to FIG. 1, the control system determines power adjustmentrequirements by cross indexing each provided LED intensity value with athermal value (already received). In an example, each provided LEDintensity value and thermal value are cross indexed in a look-up tablethat includes manufacturer provided data and/or data obtained from LEDcalibration in the factory. The resultant value obtained from thelook-up table, for each LED, is then utilized by the control system todetermine an actual contribution of each LED or independently-driven LEDgroup to the light source. Power supplied to each LED is then adjustedaccordingly.

In another embodiment and referring to FIG. 4, the control systemdetermines power adjustment requirements by cross indexing each providedsummed LED intensity value with a ratio of the component intensityvalues in a look-up table that includes manufacturer provided dataand/or data obtained from LED calibration in the factory. The resultantvalue obtained from the look-up table, for each LED orindependently-driven LED group, is then utilized by the control systemto determine an actual contribution of each LED to the light source.Power supplied to each LED is then adjusted accordingly.

The above-described apparatus and method for sensing light emittedsimultaneously from multiple light sources are example methods andimplementations. These methods and implementations illustrate onepossible approach for sensing light emitted simultaneously from multiplelight sources. The actual implementation may vary from the methoddiscussed. Moreover, various other improvements and modifications tothis invention may occur to those skilled in the art, and thoseimprovements and modifications will fall within the scope of thisinvention as set forth in the claims below.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive.

1. A light source control system comprising: at least one light source,each light source emitting a light signal at a discrete frequency and areference signal at the discrete frequency; a photodetectoroptically-coupled to the light source, the photodetector designed toreceive the light signal; and at least one lock-in system coupled to thephotodetector and each light source, each lock-in system receiving thelight signal from the photodector and receiving the reference signalfrom the light source; wherein each lock-in system produces an intensityvalue of the light source based on the light signal and the referencesignal.
 2. The apparatus of claim 1 wherein each light source comprises:a control unit; and a colored light source designed to receive a drivesignal from the control unit and produce the light signal based on thedrive signal.
 3. The apparatus of claim 2 wherein the control unit isdesigned to receive a clock signal and a power signal, produce thereference signal at the discrete frequency based on the clock signal,and produce the drive signal based on the reference signal and the powersignal.
 4. The apparatus of claim 1 wherein the photodetector comprisesa single-junction photodiode.
 5. The apparatus of claim 1 wherein theintensity value is the intensity of the light signal at the associateddiscrete frequency.
 6. The apparatus of claim 1 wherein each lock-insystem comprises: a frequency multiplier; and a filter, the filtercoupled to the frequency multiplier; wherein the intensity value is theproduct of the received light signal and the reference signal processedthrough the frequency multiplier, and filtered to remove non-dcportions.
 7. The apparatus of claim 6 wherein the filter is a low-passfilter.
 8. The apparatus of claim 1 wherein the photodetector comprisesa multi-junction photodiode.
 9. The apparatus of claim 8 wherein eachjunction of the multi-junction photodiode receives a portion of thelight signal, the portion of the light signal received based on anassociated spectra of the light signal.
 10. The apparatus of claim 9wherein the at least one lock-in system comprises a plurality of lock-indevices, each lock-in device coupled to the photodetector to receive aportion of the light signal.
 11. The apparatus of claim 10 wherein eachlock-in device comprises: a frequency multiplier; and a filter, thefilter coupled to the frequency multiplier; wherein a partial intensityvalue is produced from the product of the portion light signal receivedby the lock-in device and the reference signal processed through thefrequency multiplier, and filtered to remove non-dc portions.
 12. Theapparatus of claim 11 wherein the intensity value is the sum of thepartial intensity values.
 13. The apparatus of claim 11 wherein thefilter is a low-pass filter.
 14. A method for sensing intensity of alight source: emitting at least one light signal, each light signalemitted at a discrete frequency; transmitting a reference signalassociated with each of the light signals at the associated discretefrequency; and producing an intensity value based on the light signaland the associated reference signal.
 15. The method of claim 14 whereinemitting the light signal comprises: receiving a clock signal; receivinga power signal; and producing the light signal based on the clock signaland the power signal.
 16. The method of claim 14 wherein transmittingthe at least one reference signal comprises: receiving a clock signal;and producing the reference signal based on the clock signal.
 17. Themethod of claim 14 wherein producing the light signal comprises:receiving the light signal into a lock-in system; multiplying the lightsignal by the associated reference signal; and filtering non-dc portionsfrom the multiplied signal.
 18. The method of claim 17 wherein receivingthe light signal comprises: collecting the light signal with aphotodetector; and passing the collected light signal to the lock-insystem.
 19. The method of claim 17 wherein receiving the light signalcomprises: collecting a first portion of the light signal with a firstportion of the photodetector; collecting a second portion of the lightsignal with a second portion of the photodetector; passing the firstportion of the light signal to a first lock-in device within the lock-insystem; and passing the second portion of the light signal to a secondlock-in device within the lock-in system.
 20. The method of claim 19wherein producing the light signal further comprises: summing the firstportion of the filtered light signal and the second portion of thefiltered light signal.
 21. A system for sensing intensity of a lightsource: means for emitting at least one light signal, each light signalemitted at a discrete frequency; means for transmitting a referencesignal associated with each of the light signals at the associateddiscrete frequency; and means for producing an intensity value based onthe light signal and the associated reference signal.